1. Field of the Invention
This invention concerns liquid level detection systems in general and, more specifically, deals with an ultra sensitive liquid level detector system and/or method which is extremely simple to construct and yet highly accurate and therefore effective in particular applications of liquid level measurement for small level changes.
2. Description of the Prior Art
There are numerous patents dealing with measurement of fluid or liquid levels. Among such patents are three examples of a type where the measurement is substantially a mere indication of the presence or absence of the liquid at a given location, e.g. that of a detection instrument in the system. Examples are: U.S. Pat. Nos. 3,110,890, V. C. Westcott et al., Nov. 12, 1963; 3,256,738, S. F. Di Giacomo et al., June 21, 1966; and 4,165,641, Pomerantz et al., Aug. 28, 1979. Clearly, the type of liquid level determination in these patents deals with the mere presence or absence of the liquid at a particular location, and as such it is not relevant to the applicants' invention.
There are also prior patents such as U.S. Pat. Nos. 3,825,025, Samuel et al., July 23, 1974 and 3,889,523, Nolte, June 17, 1975. These two patents make measurements of liquid level by using a driving crystal that introduces acoustic energy into the liquid. Then, they make a determination of the response to the acoustic energy. The response is measured by a receiver element which provides for the indication of level of the liquid that transmitted the acoustic energies. While the frequencies involved in those systems, may be in the general range of frequencies which are of concern in the applicants' invention, the principle of operation is entirely different. Also, it is quite doubtful that the accuracy obtainable with those patents can approach that of the applicants' invention.
In addition, there is a U.S. Pat. No. 4,170,311, Spaw, Oct. 9, 1979, which discloses a system that measures the level of a mass of dry material. It is done with a sensing probe that acts as an antenna, the oscillation frequency of which is varied in dependence upon the depth of the material being sensed by that antenna. This has some general similarity to the system according to the applicants' invention. However, it is clearly not substantially relevant and would not be applicable to a liquid measurement of the type with which the applicants' invention is concerned.
Consequently, it is an object of this invention to provide a system and/or method for making a highly sensitive liquid level measurement making use of the natural frequency of vibration of a piezoelectric transducer.